1. Field of the Invention
The present invention relates to a technical field for using a piezoelectric piece, for example a crystal piece, to detect the magnitude of external force acting on a piezoelectric piece based on an oscillation frequency, in order to detect external force such as acceleration, pressure, flow speed of fluid, magnetic force or electrostatic force.
2. Description of the Related Art
As external force acting on a system, there are force acting on an object based on acceleration, pressure, flow speed, magnetic force, electrostatic force, and the like, and it is often necessary to measure such external force accurately. For example, in the stage of development of the automobile, there is performed measurement of impact force to seats when the automobile collides with an object. Further, there are demands for checking acceleration of vibrations or the like as accurate as possible in order to check vibration energy and amplitude during an earthquake.
Furthermore, other examples of external force measurement are checking flow speed of liquid or gas accurately and reflecting detection values thereof to a control system, measuring performance of a magnet, and the like.
To perform such measurement, there are demands for high accuracy measurement with a structure as simple as possible.
Patent Document 1 describes that a piezoelectric film is cantilever-supported, the piezoelectric film is deformed by a change in surrounding magnetic force, and an electric current flowing through the piezoelectric film changes.
Further, Patent Document 2 describes that there are provided a capacitive coupling type pressure sensor and a crystal oscillator disposed in a space partitioned from the area where this pressure sensor is disposed, a variable capacitor of the pressure sensor and the crystal oscillator are connected in parallel, and a pressure is detected by a change of an antiresonance point of the crystal oscillator by a change in capacitance in the pressure sensor.
These Patent Documents 1, 2 differ completely in principle from the present invention.
Patent Document 1: Japanese Patent Application Laid-open No. 2006-138852 (paragraph 0021, paragraph 0028)
Patent Document 2: Japanese Patent Application Laid-open No. 2008-39626 (FIG. 1 and FIG. 3)